Title: Resolving the small-scale structure of the circumgalactic medium at z~3

Abstract: Modern cosmological simulations predict that in the distant Universe the bulk of cold gas is never shock-heated, but rather cold mode accretion occurs along dense filaments of the cosmic web. Although these ``cold streams'' are postulated to give rise to Lyman limit system (LLS) absorption arising from the circumgalactic medium (CGM) of galaxies, the connection between LLSs and galaxies has been extremely difficult to establish, because of the dearth of galaxy-quasar pairs at small separations. Here we adopt a novel approach, and propose to measure the small-scale structure and clustering of LLSs. By targeting quasar pairs with projected separations 1-250 \rm kpc at z\ge3, we will measure the incidence of correlated absorption at the Lyman limit. The shape of the resulting correlation function directly tests the link between LLSs and galaxies, and simultaneously measures the covering factor of optically-thick hydrogen in the CGM of z~ 3 galaxies. These observations will also yield a large sample of LLS-pairs, constraining the degree of small-scale metal mixing and the amount of velocity shear inside galactic halos.